Seminars 2016
21.03.2016 16:00
Michal Zajaček
I. Institute of Physics, University of Cologne, Germany
An update about Dusty S-cluster object (DSO/G2) near the supermassive black hole
Abstract
Michal Zajaček
An update about Dusty S-cluster object (DSO/G2) near the supermassive black hole
I will briefly summarize our recent findings about the DSO/G2 infrared-excess object that is now in its post-peribothron phase. In particular, I will show my models that can explain the basic observational characteristics of the object. In the end, I will present a generalized model of the star-ambient medium interaction near Sgr A* and how it can be used to constrain the properties of the interstellar medium in the immediate vicinity of the supermassive black hole.
21.03.2016 15:00
Ondrej Pejcha
Department of Astrophysical Sciences, Princeton University
Death throes of binary stars: cool and luminous transients
Abstract
Ondrej Pejcha
Death throes of binary stars: cool and luminous transients
A fraction of binary stars evolve through a phase of dramatic angular momentum loss accompanied by envelope mass ejection, which might result in the merger of the two stars. The discovery of a contact binary with rapidly decreasing orbital period followed by an outburst in V1309 Sco established a connection between such catastrophic phases of binary star evolution and a group of transients characterized by their red color and the luminosity lying in the gap separating classical novae and supernovae. To utilize the wealth of information gathered on these transients, we explore the dynamics of outflows from mass-losing binary stars using smoothed particle radiation-hydrodynamic simulations with realistic equation of state and opacities. We identify several distinct outflow regimes as a function of the parameters of the perishing binary star and make the connection with the properties of the observed transients.
18.03.2016 10:00
Simone Recchi
Astronomical Institute, Academy of Sciences, Prague
New ideas on the formation of globular clusters
Abstract
Simone Recchi
New ideas on the formation of globular clusters
Several ideas have been put forward in the recent past about the formation of globular clusters (GCs), but none of them seem to be fully satisfactory. In particular it is very challenging to explain the presence of multiple populations of stars and the anticorrelations between some light elements (in particular the Na/O and Mg/Al anticorrelations) within GC. For most of the proposed scenarios, in order to fit these anticorrelation the GC must lose a very large fraction of first generation stars, but it is unclear how to disperse such a huge fraction of stars. We propose here that first generation stars are formed in a supershell, swept up after the explosion of a very massive primordial star (the so-called popIII star). Since these supershell stars are born at some distance from the center of the GC, and since they inherit the outward-directed radial velocity of the supershell, it is likely that most of them will be unbound. On the other hand, a significant fraction of the ejecta of these supershell stars can propagate, almost unimpeded, towards the center of the GC. They pile up there, cool (because of the large densities reached in the center) and can form a new generation of stars. We will present in this seminar the details of this scenario and 1-D chemo-dynamical simulations corroborating it.
29.02.2016 15:00
Attila Meszaros
Astronomical Institute, Faculty of Mathematics and Physics, Prague
Gamma-ray bursts: an overview
Abstract
Attila Meszaros
Gamma-ray bursts: an overview
The gamma-ray bursts (GRBs) were discovered already half a century ago, but until today their astrophysical meaning remains unclear. There are short and long GRBs. The long GRBs can be connected to supernovae, but for the short ones no supernova-GRB relation has been proven yet. On the other hand, a gravitational wave--short GRB relation cannot be excluded. In addition, other types of GRBs can also exist. Simply, the situation cannot be declared as satisfactory.
Because GRBs are probably the farthest objects of the Universe, and are seen also in the plane of Milky Way, they can serve as an observational test of the cosmological principle. Remarkably, these tests are against this principle. The lecture is an attempt to survey the present day situation of the topic.
30.01.2016 15:00
Joachim Wambsganss
Center for Astronomy, University of Heidelberg, Germany
Gravitational lensing: giant luminous arcs, Einstein rings and exoplanets
Abstract
Joachim Wambsganss
Gravitational lensing: giant luminous arcs, Einstein rings and exoplanets
Public lecture of the Learned Society of the Czech Republic.
Two decades ago we had no idea whether planets orbiting stars other than the Sun existed at all. Today more than 500 exoplanets have been discovered, and the field of exoplanet research has advanced to become one of the most captivating branches of astronomy. Observational techniques now aim to address questions such as what the atmosphere and weather are like on some of these planets, and to determine their global statistical properties.
25.01.2016 13:00
Ondrej Kopacek
Astronomical Institute, Academy of Sciences, Prague
Deterministic chaos in a black hole magnetosphere
Abstract
Ondrej Kopacek
Deterministic chaos in a black hole magnetosphere
Dynamics of electrically charged particles in the vicinity of a rotating black hole embedded in the external large-scale magnetic field is numerically investigated. In particular, we consider a non-axisymmetric model in which the asymptotically uniform magnetic field is inclined with respect to the axis of rotation. We study the effect of inclination onto the prevailing dynamic regime of particle motion, i.e. we ask whether the inclined field allows regular trajectories or if instead, the deterministic chaos dominates the motion. In this contribution we further discuss the role of initial condition, particularly, the initial azimuthal angle. To characterize the measure of chaoticness we compute maximal Lyapunov exponents and employ the method of Recurrence Quantification Analysis.
